This invention relates to a medium-pressure steam turbine, particularly of the single-flow type for use in a high-temperature steam turbine system having a reheater.
Such medium-pressure steam turbines are known, in which cooling steam is fed via holes in the steam chest to an annular spaced located above the rotor surface and defined by a baffle which extends axially (as defined by the axis of rotation of the rotor) from the stuffing gland on one side to an axial extension of the first guide vanes on the other side, this extension forming the shaft seal at the input end of the turbine. The rotor blades of at least the first few rows are provided with canals extending parallel to the rotor rotation axis which are located above the rotor surface and interconnect annular spaces bounded in a radial direction by the shaft seals of the adjacent guide vanes and the rotor surface and in the axial direction by successive turbine blade rows.
A medium-pressure steam turbine which receives cooling steam from the reheater prior to reheating of the steam is described by W. Traupel in "Thermal Turbo-Machines," Vol. II, 2d Edition, 1968, pages 341 to 342. The relatively low-temperature steam for cooling the rotor is fed into an annular space adjacent to the stuffing gland and formed by recesses in the housing of the inlet part and the rotor, this cooling steam in part also serving a sealing function. The baffle, closely juxtaposed to the rotor surface, separates a region from the free steam chest for the working steam in the inlet part and conducts therein part of the cooling steam along the rotor surface up to an extension of the first guide vane, this extension forming the shaft seal and the inner termination point of the baffle. In this manner the cooling steam reaches the vicinity of the first rows of blades. However, heat is transferred to the cooling steam from the in-flowing driving steam, which with its 580.degree. C. temperature is approximately 150.degree. C. hotter than the cooling steam, through the thin wall of the baffle, whereby the cooling steam is already heated up before it reaches the blades. The blade bases of the first two rows of rotor blades are formed with axial canals located above the rotor surface so that the cooling steam provides a cooler under-current at least as far as the third guide vane for reducing the temperatures of the rotor surface at these highly stressed points. With such a turbine design the rotor in high-temperature steam turbines can be fabricated from a ferritic material instead of austenitic steel which has unfavorable thermal expansion characteristics and production requirements.
An object of the invention is to provide an improved medium-pressure steam turbine of the above-described type, in which the thermally induced stress of the rotor in the region of the first rows of blades is further reduced by the additional cooling steam, in order to retain the advantage of using ferritic or martensitic materials at still higher live-steam temperatures.